Commercial processes for the manufacture of vinyl aromatic compounds such as monomeric styrene, divinyl benzene and lower alkylated styrenes (such as alphamethylstyrene and vinyltoluene) typically produce products contaminated with various impurities, such as benzene, toluene and the like. These impurities must be removed in order for the monomer product to be suitable for most applications. Such purification of vinyl aromatic compounds is generally accomplished by distillation.
However, it is well known that vinyl aromatic compounds polymerize readily and that the rate of polymerization increases rapidly as the temperature increases. In order to prevent polymerization of the vinyl aromatic monomer under distillation conditions various polymerization inhibitors have been employed.
In general, the compounds which are commercially employed as such polymerization inhibitors are of the dinitrophenolic class. Thus, for example, Drake et al, in U.S. Pat. No. 2,526,567, show the stabilization of nuclear chlorostyrenes employing 2,6-dinitrophenols. Similarly, U.S. Pat. No. 4,105,506, to Watson, discloses the use of 2,6-dinitro-p-cresol as a polymerization inhibitor for vinyl aromatic compounds.
More recently, it has been disclosed by Butler et al, in U.S. Pat. No. 4,466,905, that, in the presence of oxygen, the presence of phenylenediamines in the distillation column with 2,6-dinitro-p-cresol will further reduce the amount of polmyerization which occurs.
While dinitrophenols are effective polymerization inhibitors, there are several disadvantages associated with their use. For example, dinitrophenols are solids that, if subjected to temperatures above their melting points, are unstable and may explode (see U.S. Pat. No. 4,457,806). Thus the bulk shipment and storage of these materials as solids is precluded.
Accordingly, it is necessary to ship and store these inhibitors as a solution, preferably in a solvent compatible with vinyl aromatic monomer processing, typically a low boiling hydrocarbon solvent. Unfortunately, most dinitrophenols have low solubilities in such preferred solvents in the range of temperatures to which they are likely to be exposed during shipment and storage. For example, the solubility of 2,6-dinitro-para-cresol is about 20 percent by weight in ethylbenzene at 18.degree. C. This figure decreases rapidly as temperatures drop, with such compound being 15% soluble at -2.degree. C., 10% at -14.degree. C. and only 4.2% soluble at -25.degree. C. Once precipitated from solution, these dinitrophenol compounds do not readily return to solution, even when the temperature is returned to more normal storage temperature (e.g. of about 18.degree. C.).
Moreover, dinitrophenols are highly toxic, having an LD.sub.50 (rat) of less than 30 mg/Kg (Sax, Hazardous Properties of Industrial Chemicals).
The high toxicity and low solubility of such dinitrophenolic inhibitors coupled with the flammability of the solvents employed render the shipment and storage of solutions of dinitrophenolic inhibitors in their preferred solvents expensive and somewhat hazardous. Further, if the inhibitor precipitates from solution due to low temperatures during shipment or storage, the actual concentration may fall far below the stated concentration. If such inhibitor solution gets changed to a vinyl aromatic distillation column on the basis of its stated concentration, the low level of inhibitor actually reaching the distillation column can result in catastrophic failure of the distillation column due to explosive polymerization of the vinyl aromatic monomer.
It would, therefore, be desirable to possess a concentrated solution of dinitrophenol which thus could be more safely and economically shipped. Moreover, it would be desirable to possess a concentrated solution of dinitrophenol which would exhibit desirable low temperature stability.
Accordingly, it is an object of this invention to a provide concentrated solution of dinitrophenol, which concentrated solution possesses a lower flash point than conventional nonconcentrated solutions, and which may therefore be transported more safely.
It is another object of this invention to provide a concentrated solution of dinitrophenol which will require less volume of solvent per given amount of dinitrophenol and which may therefore be more economically shipped.
It is a further object of this invention to provide a dinitrophenol solution which exhibits desirable low temperature stability.
The foregoing and additional objects will become more fully apparent from the following description and accompanying Examples.